1. Field of the Invention
The present invention relates to improvements in a method of molding resin to seal electronic parts, such as ICs, LSIs, diodes or capacitors mounted on lead frames, for example, with thermosetting resin materials. More particularly, the invention relates to improvements for preventing electronic parts molded and sealed with resin (mold packages) from having voids formed in the interior and the exterior thereof, as well as for preventing bonding wires from suffering a wire sweep phenomenon caused by the resin, and also attaining an improvement in adhesion between the lead frames and the resin.
2. Description of the Background Art
In general, an electronic part is molded and sealed with resin by a transfer mold method, which is generally carried out using a resin sealing/molding apparatus, for example, in the following manner.
A fixed upper mold section and a movable lower mold section of the resin sealing/molding apparatus are initially heated to a resin molding temperature by heating means and are then opened. An apparatus having an inverse structure, namely a movable upper mold section and a fixed lower mold section is also known.
Then, a lead frame carrying an electronic part is set at a prescribed position on a die surface of the lower mold section, while a resin tablet is supplied into a pot of the lower mold section.
Then, the lower mold section is upwardly moved so that the upper mold section and the lower mold section are closed. Thus, the electronic part and the lead frame carrying the part are set between upper and lower cavities that are provided opposite one another in the die surfaces of the lower mold section and the upper mold section, while the resin tablet in the pot is heated so that it gradually melts.
Then, the resin tablet in the pot is pressurized by a plunger so that the melted resin material is injected or charged into the cavities, whereby the electronic part and the lead frame set in the cavities are sealed in a resin molded member, which is molded to have a shape corresponding to the shapes of the cavities.
After a lapse of time required for hardening the melted resin material, the upper mold section and the lower mold section may be opened so as to release the resin molded member and the lead frame from the cavities while releasing the hardened resin from a resin passage by means of a mold releasing mechanism.
When the upper mold section and the lower mold section are closed, air containing moisture such as that in the atmosphere, for example, remains in a space defined by the pot, the resin passage and the cavities. Such residual air may enter the melted resin material to cause problems during resin molding such as the formation of voids in the interior and the exterior of the resin molded member.
In order to cope with this problem, the cavities are made to communicate with the exterior through proper air vents and the melted resin material held in the pot is transferred through the resin passage and injected or charged into the cavities, so that the aforementioned residual air and the like are naturally discharged to the exterior through the air vents.
While the air remaining in the pot, the resin passage and the cavities can be naturally discharged through the air vents as hereinabove described, it is impossible in practice to effectively inhibit the formation of internal voids and outer surface defects in the resin molded member. Thus, it is impossible to reliably solve the problem of deterioration in moisture resistance and appearance of the product.
When internal voids are formed in the resin molded member, the adhesion between the lead frame and the resin is deteriorated, and moisture easily infiltrates into the product through the clearances or gaps between the resin and the lead frame, which deteriorates the moisture resistance of the product, for example. Further, the resin molded member is easily cracked or chipped by forces that are applied to bases or ends of outer lead portions projecting from the resin molded member whereby the lead ends are bent. Thus, it is impossible to attain the high quality and high reliability that are absolutely required for this product.
The internal voids that are formed in the resin molded member mainly result from at least one of air and moisture contained in the resin tablet. The resin tablet, which is formed by merely compacting resin powder into a prescribed shape, generally contains a large quantity of air, which further contains moisture in the atmosphere. Furthermore, the large quantity of air contained in the resin tablet is present in the form of a number of independent bubbles, which are isolated from each other, and which cannot communicate with each other and with the surface of the resin tablet for ventilation or expulsion from the tablet.
When the resin tablet is heated to a resin molding temperature of at least 175.degree. C., for example, the air or moisture contained in the resin tablet forms water vapor, which enters the melted resin material with the air remaining in the aforementioned internal die space. Thus, the resin material is injected or charged into the cavities in this state, and hence voids are formed in the interior and the exterior of the resin molded member due to the air or moisture contained in the resin tablet.
Further, gas such as combustion gas, which is generated upon heating of the resin tablet, also serves as a cause for forming voids in the interior and the exterior of the resin molded member.
In order to prevent the formation of voids in the interior and the exterior of the resin molded member, the following counter-measure may be employed. For example, the air remaining in the aforementioned internal die space between the upper mold section and the lower mold section may be discharged to the exterior by a vacuum pump, thereby preventing the residual air from entering the melted resin material.
When ordinary air discharge means such as the vacuum pump is employed, however, the overall time duration of the resin sealing/molding process is disadvantageously lengthened, because a long time is generally required for increasing the degree of vacuum in the internal die space to a prescribed or sufficient level.
In general, an electronic part is molded and sealed with a thermosetting resin such as epoxy resin, for example, and hence the resin tablet that is supplied into the aforementioned internal die space (pot) is hardened with time after the same is heated and melted.
When the internal die space is evacuated for a long time using ordinary air discharge means, therefore, the melted resin material undergoes hardening which deteriorates its flowability. Thus, the evacuating operation is preferably carried out in a short time in consideration of this point.
As hereinabove described, gas such as combustion gas that is generated upon heating of the resin tablet also forms voids and defects in the interior and the exterior of the resin molded member. Such gas is generated by a coupling agent, a mold release agent, a flexibility agent or other additive agents contained in the resin tablet, which vaporize when the resin tablet is heated to a resin molding temperature of 175.degree. C., for example, and melted.
In another conventional countermeasure for preventing the formation of voids in the resin molded member, the melted resin material which is injected into the cavities is pressurized to have an increased resin pressure for compressing voids contained therein, thereby compressing voids formed in the interior of the resin molded member to a negligible degree.
However, the conventional method of compressing the voids under a high pressure has the following problems:
(1) This method is disadvantageous in consideration of the durability of the apparatus, dangerousness in operation, the cost and other known disadvantages, due to the requirement for a large-sized press machine which can carry out high-pressure compression molding. PA0 (2) Viscous resin which starts to be hardened in the cavities in a final stage of the compression step is made to flow in such high-pressure compression molding. Thus, a bonding wire for electrically coupling an electrode of a semiconductor element and the outer lead portion is inclined or swept toward the direction of resin flow in the so-called wire sweep phenomenon. This may lead to serious problems such as contact between the bonding wire and an adjacent one, disengagement of a bonding pad wire from the electrode, or wire breaking.
In the aforementioned conventional countermeasure for preventing the formation of voids, further, it is impossible to make effective use of multistage injection for preventing the problem of the bonding wire sweeping phenomenon, electrode disengagement, wire breaking or other known disadvantageous phenomena. This problem is now described with reference to FIG. 5.
In a conventional method of injecting a melted resin material into cavities without multistage injection, a plunger engaged in a pot 109 is first upwardly moved to pressurize and inject a melted resin material through a cull portion 119 and a gage 121 into cavities 110 and 120 which are oppositely formed in a lower mold section 108 and an upper mold section 118, as shown in FIG. 5. The speed of injecting the resin in the cavities 110 and 120 can be adjusted by controlling the speed of upwardly moving the plunger.
If the resin injection speed is high, the resin is injected into the cavities 110 and 120 to spout from a port 121a. Thus, the injected resin entrains air which is present around the port 121a, to form voids and defects in the vicinity of the port 121a. Further, a spiral flow of the resin is caused in the cavities 110 and 120 and entrains air remaining therein to form voids and defects, while causing a wire sweep phenomenon of a bonding wire 152, electrode disengagement, wire breaking or other known disadvantageous phenomena.
When multistage injection, i.e., a molding method of adjusting the speed or pressure of injecting the melted resin material into the cavities 110 and 120 under constant conditions, is employed, on the other hand, the following action is obtained. The speed of injecting the resin through the port 121a is relatively reduced during a time and in a range (A in FIG. 5) where the resin is completely charged around the port 121a, and during a time and in a range (C in FIG. 5) where the resin is charged around the wire 152. In the remaining ranges (B and D in FIG. 5), the resin is injected at an ordinary injection speed, which is higher than that in the ranges A and C.
In this case, it is possible to suppress spouting of the resin that is injected from the port 121a into the cavities 110 and 120 by reducing the injection speed therefor in the range A shown in FIG. 5, thereby preventing the injected resin from entrainment of air and occurrence of a spiral flow in the vicinity of the port 121a.
In the range B shown in FIG. 5, on the other hand, it is possible to inject the resin at an ordinary injection speed since no bonding wire 152 is provided in this range B. While the resin is injected at the ordinary injection speed also in the range A at this time, neither entrainment of air nor occurrence of a spiral flow is caused in the injected resin in the vicinity of the port 121a since the range A is already charged or filled with the resin.
Then, the injection speed for the resin is reduced, because the bonding wire 152 is provided in the range C shown in FIG. 5, so as to prevent the front surface of the resin being injected into the cavities 110 and 120 from forcibly coming into contact with and thus bending the bonding wire 152. In this manner, it is aimed to prevent a wire sweep phenomenon of the bonding wire 152, electrode disengagement and wire breaking caused by such bending action.
Then, the resin can be injected at an ordinary injection speed in the range D, which contains no bonding wire 152. While the resin is injected at the ordinary injection speed also through the ranges A to C at this time, neither entrainment of air nor occurrence of a spiral flow is caused in the injected resin in the vicinity of the port 121a since the ranges A to C are already charged or filled with the resin.
By using such multistage injection, it is possible to solve the aforementioned problems of formation of voids and defects, a wire sweeping phenomenon of the bonding wire 152 and other problems which are caused by the resin spouting from the port 121a into the cavities 110 and 120.
The aforementioned countermeasure for preventing the occurrence of voids by high-pressure compression molding can be compared to multistage injection by the following features. The aforementioned high-pressure compression molding is effective for preventing the occurrence of voids, but suffers the problems of a wire sweep phenomenon of a bonding wire, electrode disengagement and wire breaking. While multistage injection can prevent a wire sweep phenomenon, electrode disengagement and wire breaking, it is not effective for preventing the occurrence of voids.
It may be possible to carry out high-pressure compression molding after carrying out multistage injection. However, the aforementioned problems following high-pressure compression molding cannot be solved by such means. Consequently, it is impossible to make effective use of multistage injection when the conventional countermeasure of high-pressure compression molding is used for preventing occurrence of voids.